1. Field of the Invention
The present invention relates to a fixing apparatus and an image forming apparatus including the fixing apparatus.
2. Description of the Related Art
Hitherto, a heat roller system has been frequently used as a heating apparatus for an image forming apparatus. In addition, a heating apparatus of a film heating system has been put into practical use in recent years from the viewpoints of a quick start and energy saving.
Available as a fixing apparatus of the film heating system is an apparatus including a heater including a resistance heating element on a substrate made of ceramics, a flexible member (fixing belt) that moves while being in contact with the heater, a sliding pressure member, and a pressure roller.
The sliding pressure member is disposed inside the fixing belt and is pressurized while sliding on an inner surface of the fixing belt. The pressure roller forms a nip portion with the sliding pressure member through the fixing belt. A recording material bearing an unfixed toner image is heated while being sandwiched and conveyed at the nip portion of the fixing apparatus. Thus, the image on the recording material is heated and fixed onto the recording material.
The fixing apparatus has the advantage that it requires only a short time to attain a temperature for image fixation after the start of supplying electric power to the heater. Accordingly, a printer mounted with the fixing apparatus can shorten a time period required from input of a print command to output of the first image. In addition, the fixing apparatus of this type has the advantage that power consumption is small while it is ready and waiting for the print command.
A heat capacity of the film serving as a heating member of the fixing apparatus of the film heating system is smaller than that of a fixing apparatus of the heat roller system. Accordingly, regulation of the heater temperature and control of paper feeding have been performed according to the size and kind of the recording material onto which an image is to be fixed.
However, in such fixing apparatus of the film heating system having a low heat capacity, the sliding pressure member is pressurized while sliding on the inner surface of the fixing belt, and hence a rotational torque occurs. In general, sliding grease or the like is applied to the inside of the fixing belt to reducing the rotational torque.
However, when the sliding grease deteriorates over time, or is thermally decomposed or depleted, the rotational torque increases.
When the rotational torque increases, faulty rotation of the fixing belt is liable to occur, and hence abnormal noise due to a stick-slip phenomenon or faulty images called an image slip due to delayed paper conveyance may occur. Measures given below have been known as measures against such increase in the torque of the fixing apparatus.
Japanese Patent Application Laid-Open No. 2003-57978 discloses a heater whose surface coming into contact with a fixing belt is coated with a sliding layer having a thickness of 10 μm or less. An imide-based resin such as polyimide or polyamide imide, a fluorine-based resin, or the like has been used as a material for the sliding layer. Examples of the fluorine-based resin include a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) and polytetrafluoroethylene (PTFE).
In addition, Japanese Patent Application Laid-Open No. 2009-58661 discloses an invention which aims to improve quality of a fixed image by: imparting conductivity to a heat-resistant resin belt that forms an image forming apparatus to reduce static electricity; and improving abrasion resistance of a film to effectively prevent the film from peeling. It is also disclosed that the aim can be achieved by a heat-resistant resin belt having formed on its inner circumferential surface a coating layer having a Vickers hardness of 3,000 Hv or more and a surface resistance value of 105Ω·cm or less.
Japanese Patent Application Laid-Open No. 2009-58661 discloses, as an example of such coating layer, a diamond-like carbon film (hereinafter sometimes referred to as “DLC film”) having a Vickers hardness of 3,000 Hv or more and a surface resistance of 102Ω·cm. In addition, Japanese Patent Application Laid-Open No. 2009-58661 discloses, as an example of the coating layer, a tetrahedral amorphous carbon film (hereinafter sometimes referred to as “ta-C film”) having a Vickers hardness of about 5,000 Hv and a surface resistance of 102Ω·cm. It should be noted that the ta-C film is one kind of the DLC film and is an amorphous carbon-based hard thin film free of hydrogen.
As a result of an investigation, the inventor of the present invention has recognized that when the endless heat-resistant resin belt provided with the DLC film or ta-C film as the coating layer according to Japanese Patent Application Laid-Open No. 2009-58661 is applied to a fixing member to be placed in such a state that the coating layer slides on a pressure member, such problems as described below occur.
That is, when the coating layer formed of the DLC film or ta-C film having such flexibility as to be capable of following the bending property of the resin belt is placed in such a situation that the pressure member always slides on the coating layer, its durability has not yet been sufficient. Accordingly, when the fixing apparatus is used over a long period of time, a frictional resistance between the inner circumferential surface of the endless heat-resistant resin belt and the pressure member increases owing to, for example, peeling of the coating layer, and hence the rotational torque of the heat-resistant resin belt may increase, or stick-slip (sticking) may occur in some cases. In addition, abnormal noise or faulty electrophotographic images resulting from the foregoing phenomena may occur in some cases.